Hold the presses!! I have checked four cassettes so far and 3 of them have curled tape edges giving horrible wow and flutter.
It could be of course that these tapes were damaged, left in strong sunshine for example but it could equally well be that the tape degrades in some way over that many years? If that is the case then anyone buying a pack of such cassettes advertized as "NOS" should immediately unpack and check them on a recorder.

I have seen them on Amazon for 48.00 +p&p for the same shrink wrapped 5 pack so we are talking a few bob here!

Mine only cost me 5 quid so if I get just two good'uns I am winning.